This invention relates to fuel cell stacks having external manifolds and, in particular, to an external manifold system for use in coupling gases to or from the face of a fuel cell stack.
A fuel cell is a device which transforms chemical energy in the form of fuel (e.g., natural gas, bio-gas, methanol, diesel fuel, etc.) directly into electrical energy by way of an electrochemical reaction. Like a battery, a fuel cell contains two electrodes, an anode and a cathode. Unlike a battery the fuel cell will produce electrical power as long as fuel and oxidant are delivered to the anode and cathode, respectively. The major advantage of fuel cells over more traditional power generation technologies (e.g., IC engine generators, gas or steam turbines, etc.) is that the fuel cell converts chemical to electrical energy without combusting the fuel. The efficiency of the fuel cell is, therefore, not thermodynamically limited, as are heat engines, by the Carnot cycle. This allows fuel cell based systems to operate at a far higher efficiency than traditional power plants thereby reducing fuel usage and byproduct emissions. Additionally, due to the controlled nature and relatively low temperature of the chemical reactions in a fuel cell, the system produces nearly zero pollutant emissions of hydrocarbons, carbon monoxide, nitrogen oxides and sulfur oxides.
Fuel cells are typically arranged in stacked relationship. A fuel cell stack includes many individual cells and may be categorized as an internally manifolded stack or an externally manifolded stack. In an internally manifolded stack, gas passages for delivering fuel and oxidant are built into the fuel cell plates themselves. In an externally manifolded stack, the fuel cell plates are left open on their ends and gas is delivered by way of manifolds or pans sealed to the respective faces of the fuel cell stack. The manifolds thus provide sealed passages for delivering fuel and oxidant gases to the fuel cells and directing the flow of such gases in the stack, thereby preventing those gases from leaking either to the environment or to the other manifolds. The manifolds must perform this function under the conditions required for operation of the fuel cell and for the duration of its life.
An important aspect of the performance of a fuel cell stack manifold is the gas seal established between the manifold edge and the stack face. As the stack face is typically electrically conductive and has an electrical potential gradient along its length and the manifold is typically constructed from metal, a dielectric insulator is needed to isolate the manifolds from the fuel cell stack and prevent the manifolds from shorting the stack. The dielectric insulator is typically constructed from ceramic, which tends to be brittle; therefore, manifold compression against the stack face or other mechanical changes in the manifolds due to thermal or mechanical stresses on the manifold system during operation of the fuel cell stack may damage the dielectric insulators.
Another requirement of fuel cell stack manifolds relates to the fact that typically a fuel cell stack will shrink over its life as the cell components creep and densify at high temperature. For a tall fuel cell stack (of approximately 300 fuel cells or more) the total height may decrease by 2-3 inches. This means that continuous metal manifolds cannot be fixed to both the top and bottom of the stack but rather must be able to accommodate such changes in stack dimensions during operation. Therefore, the manifold system employed to direct gas flows in the fuel cell stack must be flexible enough to move with the stack but must also maintain the gas seal. In addition, as discussed above, the stresses on the manifold system during operation of the stack must be at least partially absorbed so that the ceramic dielectric insulators are not caused to break.
Due to manufacturing defects before operation and due to its inherently non-uniform temperature distribution during operation, a tall fuel cell stack tends to bow. Horizontal deflection of the top of the stack at high temperatures can be as much as 1-2 inches relative to the base of the stack. This places a further burden on the manifolds, which are required to flex with the bowing stack in order to maintain tight gas seals.
Fuel cells operate at temperatures above ambient (Polymer Electrolyte Fuel cells, “PEFC”: operate at about {tilde over ( )}80° C.; Phosphoric Acid Fuel cells, “PAFC”: operate at about {tilde over ( )}200° C.; Molten Carbonate Fuel cells, “MCFC”: operate at about {tilde over ( )}650° C.; Solid Oxide Fuel cells, “SOFC”: operate at about {tilde over ( )}1000° C.). Therefore, the selection of materials and the mechanical design must allow the components to last for the life of the fuel cell stack (typically years). Component stress and corrosion must be considered relative to the environment in which these components must perform. In the case of MCFC and SOFC the temperatures are high enough and the lifetime long enough that long term creep of metallic components must be considered in their design.
The fuel cell manifold system currently used by the assignee of the subject application for tall carbonate fuel cell stacks is of a type as generally shown in FIG. 1A and includes solid rails positioned along the length of the stack and a manifold body comprising a pan. Dielectric insulators are typically fixed to the manifold by woodruff keys. This manifold system is somewhat more effective on short stacks (approximately 40 fuel cells or less) due to proportionately less bowing and deflection of the stack. The components used in this type of manifold system are constructed from high-temperature, corrosion-resistant materials such as nickel-based alloys and stainless steels.
The aforesaid manifold system also operates in conjunction with a retention system having a large quantity of different parts to satisfy the requirements for a uniformly distributed normal load application to the manifold that maintains the manifold in sealing relationship to the stack as well as allows both stack shrinkage and stack bowing. The selected materials, intricacy of the geometry and large number of parts used in this design make it expensive, heavy and difficult to install. Also, the retention system currently used by the assignee of this application in conjunction with the manifold system is designed to function completely independently from the manifolds and thus results in certain redundancies of material that add to the cost, weight and complexity of the fuel cell stack.
It is therefore an object of the present invention to provide a fuel cell stack manifold system that does not suffer from the above disadvantages.
It is a further object of the present invention to provide a fuel cell stack manifold system which is less costly, less complex and easier to manufacture.
It also an object of the present invention to provide a fuel cell stack manifold system that is flexible to accommodate stack bowing and shrinkage while maintaining a gas seal.
It is another object of the present invention to provide a fuel cell stack manifold system that includes a dielectric fixture arrangement that reduces or eliminates dielectric insulator breakage.
It is yet another object of the present invention to provide a fuel cell stack manifold system which requires few components and results in more effective gas sealing.